An electrophoretic cell is a cell comprised of pigment particles suspended in a fluid and uses electrophoresis to switch between the following two states:
Distributed State: Particles are positioned to cover the horizontal area of the cell. This can be accomplished, for example, by dispersing the particles throughout the cell, by forcing the particles to form a layer on the horizontal surfaces of the cell, or by some combination of both.
Collected State: Particles are positioned to minimize their coverage of the horizontal area of the cell, thus allowing light to be transmitted through the cell. This can be accomplished, for example, by compacting the particles in a horizontal area that is much smaller than the horizontal area of the cell, by forcing the particles to form a layer on the vertical surfaces of the cell, or by some combination of both.
The electrophoretic cell can serve as a light valve since the distributed and collected states can be made to have different light absorbing and/or light scattering characteristics. As a result, an electrophoretic cell can be placed in the light path between a light source and a viewer and can be used to regulate the appearance of a pixel in a display.
Transmissive color displays are known that use liquid crystals and crossedpolarizers to control the intensity of the light through the color filters in each pixel. The use of linear polarizers limits the transmission of light through the display and, hence, the backlight efficiency, brightness and power efficiency of these displays is reduced. These displays also suffer from limited viewing angle. Electrophoretic displays can offer improved transmission characteristics combined with extremely low power requirements.
Electrophoretic color displays have been proposed in the prior art. Evans, et al., in U.S. Pat. No. 3,612,758, describe a two-color electrophoretic display having pigment particles of a single color in a contrasting dye solution. In this scheme, under the influence of an electric field, the particles migrate to a front transparent electrode and the viewer sees the color of the particles. When the field is reversed, the particles migrate away from the front transparent electrode, are hidden in the dye solution, and the viewer sees the color of the dye solution.
Japanese Patent No. JP 1267525 assigned to Toyota Jidosha KK describes a electrophoretic color display having colored (blue and yellow) particles with different zeta potentials in a solution of red dye to give a multicolored (yellow, green and red) display. When a certain voltage is applied to the pixels, the yellow particles are pulled to the front transparent electrode and the viewer sees yellow. At a higher voltage, the blue particles are also pulled to the front electrode and the viewer sees green. When the particles are pulled off the transparent electrode, the colors of the particles are hidden by the dye solution and the viewer sees red.
Hou, in WO 94/28202, describes electrophoretic color display with a dispersion comprised of two differently colored particles that are oppositely charged. The polarity of the voltage applied to the cell determines the polarity of the particle attracted to the front transparent electrode, and hence determines the color seen by the viewer.
In the electrophoretic color display references cited above, the use of a backlight is not suggested nor would these embodiments have contrast in a transmissive mode of operation. Transmissive electrophoretic displays based on backlit operation, however, have been proposed in the prior art.
Dalisa, et. al., in U.S. Pat. No. 4,218,302, describe a transmissive electrophoretic display that uses particles to either allow or frustrate the total internal reflection of light at the interface between the interior of the rear window and the suspension fluid. In the regions of this interface where no particles are present, the conditions for total internal reflection are satisfied, and light from the source is reflected back towards the source and the viewer sees no light. In the regions of this interface where the particles are present, the conditions for total internal reflection are frustrated, and light from the source passes through the interface and the viewer sees light.
Marshall et al., in U.S. Pat. No. 4,648,956, describes several embodiments of a transmissive electrophoretic display. In these embodiments the suspension is contained between a transparent, large-area upper electrode and a small-area lower electrode. When the absorbing particles in a selected region of the display are collected on the small-area lower electrode, they cover only a small portion of the horizontal area of the selected region. As a result, most of the light from the source passes through the selected region without being absorbed and the viewer sees light. When the absorbing particles are drawn to the upper electrode, they cover substantially the horizontal area of the selected region so that most of the light from the source is absorbed before it can reach the viewer and the viewer sees dark.
Hou, in U.S. Pat. No. 5,298,833, describes a transmissive electrophoretic display based on a conductive mesh screen disposed between the backlight and the viewer. The mesh screen covers the viewing area of the display and is immersed in a suspension comprised of black particles in a clear fluid. Particles are either drawn to, or removed from a selected area of the mesh by using patterned transparent electrodes disposed above and below the mesh. When the black particles are drawn to the mesh they sit on the mesh without completely covering its holes. Light from the source is transmitted through the holes and the viewer sees light. When the particles are removed from the mesh, they are drawn to cover the selected transparent electrode. Light from the source is substantially absorbed in the area covered by the electrode and the viewer sees dark.
There is a continuing need in the art for a low-power electrophoretic color display with high brightness, wide viewing angle, and large color gamut. It is therefore an object of the present invention to provide a transmissive electrophoretic color display possessing these characteristics. Other objects and advantages will become apparent from the following disclosure.